Isobutene (also referred to as isobutylene or 2-methylpropene) is a hydrocarbon of significant industrial importance. Isobutene is used as an intermediate in the production of a variety of products. For example, it is reacted with methanol and ethanol in the manufacture of the gasoline oxygenates methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE), respectively. Alkylation with butane produces isooctane, another fuel additive. Isobutene is also used in the production of methacrolein. Antioxidants such as butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) are produced by Friedel-Crafts alkylation of phenols using isobutylene.
Polymerization of isobutene with isoprene produces butyl rubber, a random copolymer of isobutene and isoprene, which is well known for its excellent thermal stability, ozone resistance and desirable dampening characteristics. Currently butyl rubber is industrially produced utilizing isobutene derived from petrochemical source(s). Isobutene used in industrial applications typically is prepared as a by-product of conventional industrial dehydration processes such as, thermal cracking process in petroleum, refining the purification of which is a multistep energy intensive process. The amount of isobutene produced varies depending on the composition of the petrochemical feedstock and the type of cracking used in the process. The stream is typically characterized by a high butadiene content and low amount of butene. After separating the butadiene, the remainder of the stream contains less than 50% isobutene. The volatility of oil prices has made petro-based feedstock of isobutene unreliable while the cracking of lighter crude has seen the overall percentage of isobutene in the C4 stream drop dramatically.
As petrochemically derived isobutene is obtained from complex hydrocarbon mixtures, it is usually necessary to carry out a further extensive (and expensive) purification prior to polymerization. Multistep processes for purification are energy and resource intensive. Accordingly, processes capable of directly providing relatively pure isobutene which require little or no additional purification would be desirable.
There is increasing environmental concern that the use of petroleum-derived hydrocarbons as basic raw materials (e.g., butadiene or isoprene) contributes to environmental hazards such as global warming and pollution and fosters overdependence on unreliable petroleum supplies. These concerns increase demand for environmentally friendly processes and products. Accordingly, there is a need for a low carbon footprint solution to produce isobutene-based polymers utilizing renewable (i.e., biologically derived) sources of monomers such as isobutene and low energy chemical processes.
There is also a concern that future supplies of isobutene from petrochemical based sources will be inadequate to meet projected needs and that prices will rise to unprecedented levels. Accordingly, there is a current need to procure a source of feedstock material, such as isobutene from a low and reliable cost, renewable source which is environmentally friendly.
U.S. Application No. 12/711,919 (published as US 2010/0216958 A1) discloses use of isobutanol obtained from renewable sources in the preparation of isoprene, butadiene and isobutene. In the process disclosed therein, isobutene is separated from 1,3-butadiene using acetonitrile extraction, following a dehydrogenation (400° C., two catalyst system—ZnFe2O4 and Co9Fe3BiMoO51) of the linear butenes (1-butene, cis- and trans-butene). The separation yields isobutene with residual linear butenes and trace amounts of butadiene. Linear butenes and 1,3-butadiene are known to act as potent poisons/chain transfer agents in the cationic polymerizations of butyl rubber. Parts per million (ppm) quantities could affect the molecular weight of the resultant polymer, therefore high purity isobutene feeds are required. The initiators/co-initiator used for the polymerization, particularly for preparing butyl rubber is highly sensitive to impurities.